A first-of-its-kind global map shows forest canopy height in shades of green from 0 to 70 meters (230 feet). For any patch of forest, the height shown means that 90 percent or more of the trees in the patch are that tall or taller. Areas without forest are shown in tan. Credit: NASA Earth Observatory/Image by Jesse Allen and Robert Simmon/Based on data from Michael Lefsky.
Temperate conifer forests -- which are extremely moist and contain massive trees such as Douglas fir, western hemlock, redwoods, and sequoias--have the tallest canopies, soaring easily above 40 meters (131 feet). In contrast, boreal forests dominated by spruce, fir, pine, and larch had canopies typically less than 20 meters (66 feet). Relatively undisturbed areas in tropical rain forests were about 25 meters (82 feet), roughly the same height as the oak, beeches, and birches of temperate broadleaf forests common in Europe and much of the United States.
A forest canopy height map of the contiguous United States. Credit: NASA Earth Observatory/Image by Jesse Allen and Robert Simmon/Based on data from Michael Lefsky.
Scientific interest in the new map goes far beyond curiosities about tree height. The map has implications for an ongoing effort to estimate the amount of carbon tied up in Earth’s forests and for explaining what sops up 2 billion tons of “missing” carbon each year.
Humans release about 7 billion tons of carbon annually, mostly in the form of carbon dioxide. Of that, 3 billion tons end up in the atmosphere and 2 billion tons in the ocean. It’s unclear where the last two billion tons of carbon go, though scientists suspect forests capture and store much of it as biomass through photosynthesis.
“What we really want is a map of above-ground biomass, and the height map helps get us there,” said Richard Houghton, an expert in terrestrial ecosystem science and the deputy director of the Woods Hole Research Center.
One of Lefsky’s colleagues, Sassan Saatchi of NASA’s Jet Propulsion Laboratory, has already started combining the height data with forest inventories to create biomass maps for tropical forests. Complete global inventories of biomass, when they exist, can improve climate models and guide policymakers on how to minimize the human impact on climate with carbon offsets.
More immediately, said University of Maryland remote sensing expert Ralph Dubayah, tree canopy heights can be plugged into models that predict the spread and behavior of fires, as well as ecological models that help biologists understand the suitability of species to specific forests.